The invention relates to a device for controlling the quantizer of a hybrid coder by which the pictures of a video signal are coded and quantized in data blocks and in which the quantizer is controlled in dependence upon the filling level of a buffer memory.
Such devices are applicable, for example, in picture telephony systems for obtaining video pictures of adequate quality at the receiver end by controlling the quantizer of a hybrid coder at a possibly low transmission bit rate (for example 64 kbit/s).
A coding procedure suitable for hybrid coders is described in greater detail in CCITT Recommendation H.261 (compare, for example: Draft Revision of Recommendation H.261: Video-codec for Audiovisual Services at p.64 kbit/s. Signal Processing: Image Communication 2 (1990) 221-239, Elsevier Science Publishers B.V.). In this Recommendation the components required for implementation of the method and their functions are also described. An embodiment of a hybrid coder or decoder corresponding to the H.261 Recommendation is described in the document "Description of Reference Model 7", Specialist Group on Coding for Visual Telephony, CCITT SGXV Working Party XV/4, doc. 446, 1989. The first-mentioned document will hereinafter be referred to as (D1) and the second-mentioned document will hereinafter be referred to as (D2).
According to (D1) a hybrid coder consists of a source coder which comprises a quantizer, a multiplex coder which codes the main and sub-information components supplied by the source coder into binary code words and combines them to a serial data stream, and a subsequent buffer memory which is succeeded by a line coder.
A coding control means controls the source coder and the multiplex coder in dependence upon the filling level of the buffer memory.
In this connection the name hybrid coder is based on the simultaneous use of two coding principles, namely the interframe principle and the intraframe principle. In the interframe principle the time dependence of the video pictures is utilized and in the intraframe principle the local dependence within the video pictures is utilized.
If the buffer memory is about to run empty or overflow, the quantization step (magnitude of the quantization intervals) is rendered finer or coarser in order that a larger or smaller number of data respectively occur. The control is realized in such a way that the quantization step remains constant within a data block of a given size (macroblock).
In this connection data block is understood to mean data of a section of a video picture. This section is, for example, a square part of the visible video picture. In (D1) (also compare EP 029 085 A2) four types of data blocks are distinguished:
a) blocks consisting of 64 numerical values which either represent the luminance values of a square picture section consisting of 8.times.8 pixels or one of the two chrominance components of a square picture section of 16.times.16 pixels, PA1 b) macroblocks consisting of four luminance and two chrominance blocks which jointly represent all data of a quadratic picture section of 16.times.16 pixels, PA1 c) block groups consisting of 33 macroblocks, PA1 d) video pictures consisting of 396 macroblocks each. PA1 a) Means by which the difference between two consecutive data blocks of a sequence of equivalent data blocks is determined and by which a background index L is increased by one unit when the difference falls below a predetermined value, and by which the background index L is set to its initial value whenever the difference is above the predetermined value; PA1 b) means with which the quantizer, unlike the buffer control for transmitting a representative block for a sequence of equivalent data blocks, is set to a finer quantization whenever the associated background index L has a value which is above a threshold value L.sub.max and whenever at least one further system parameter has reached or exceeded a predetermined threshold; PA1 c) means with which the quantizer, unlike the buffer control for transmitting the data blocks subsequent to the representative block of a sequence of data blocks, is set to a quantization which is at least as coarse as the quantization effected by the buffer control, as long as the background index L is above the value L.sub.max.
The term data block will hereinafter be utilized in its smaller sense according to (D1) and in its wider sense, in which the relevant picture sections may have an arbitrary size and shape.
According to (D1) a discrete cosine transform is performed with the blocks in the source coder. This leads to a bit rate reduction based on the geometrical (or rather two-dimensional) correlations which exist within a video picture and represents a concrete form of the above-mentioned intraframe principle.
The time dependence of consecutive video pictures is utilized, for example in that the difference between data is generated and further processed by means of equivalent pixels of two consecutive video pictures. Pixels or data blocks of video pictures which are different from each other will be referred to as equivalent if they have the same coordinates or addresses, i.e. if they have the same location within a video picture.